Halide Mixing Research Articles

How Halide Sub-Lattice Affects Li Ion Transport in Antiperovskites

Li-based antiperovskites (LiAP, Li3-x OH x X, X = Cl, Br) are an emergent class of Li-ion conductors that are potential candidates for electrolytes in all-solid-state batteries. As a material class, pLiAP shows vast compositional design freedom; however, the resulting properties are susceptible to synthesis and processing methodologies. For example, proton incorporation and halide mixing stabilize the perovskite cubic phase near room temperature, and halides mixtures near the eutectic points drive the solid-state reaction temperature down, allowing for faster synthesis and processing conditions (< 1 h). The mixed halogen compositions, such as Li2OHCl0.37Br0.63, also show a 30-fold improvement in room temperature ionic conductivity of a single halide structure, 1.5 x 10-6 vs. 4.9 x 10-8 S cm-1 (Li2OHCl). Despite the growing interest in these materials, important questions remain about LiAPs on the structure-property correlation upon halide substitution and the correlations between the OH/halide dynamics and the Li-ion transport. We thus attempted to deconvolute how proton dynamics and halide substitution enhance or impede ionic conduction in pLiAP at compositions near the halide salts' eutectic points.We combined infrared spectroscopy and nuclear magnetic resonance (NMR) with first-principles density functional theory (DFT) calculations to deconvolute halide mixing effects from local proton dynamics on Li-ion transport. The NMR results and ab initio molecular dynamics suggest that Li+ transport is more strongly correlated with halide dynamics. While the hydroxide does stabilize the highly conductive cubic structure, it limits correlative ionic transport and thus lowers Li+ conductivity.Experiment design, data analysis, and manuscript preparation (RLS) were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering. Synthesis (THB and JN) were supported by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program. P. J. acknowledges partial support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-96ER45579. H. F. was supported from U.S. Department of Energy (Award No. DE-EE0008865). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The NMR characterization part of the work is supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, and Basic Energy Sciences. The NMR work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research, located at Pacific Northwest National Laboratory. Figure 1

Unraveling the Effect of Halogen Ion Substitution on the Noise of Perovskite Single-Crystal Photodetectors.

Halide mixing in perovskites has been an efficient way to engineer the bandgap, stability, and charge carrier mobility of lead halide perovskites, while its effect on the noise of lead halide perovskite photodetectors is still unknown. We present the preparation of mixed halide methylammonium lead perovskite single crystals by the solution temperature-decreasing method. Planar-structured photodetectors were constructed on the basis of these mixed halide perovskite single crystals. The effect of halogen ion substitution on the noise of devices was investigated by analyzing their dark current spectra. It is shown that the single-crystal photodetectors with higher levels of chloride suffer from larger noise thus have lower detectivity. Density functional theory calculations have also been proposed to reveal the effect of halogens on band structure. These results provide a comprehensive understanding of mixed halide perovskites and may help in the design and preparation of higher-performance devices.

Trihalide Mixing by Size-Flexible H– Ions in Layered Ba2H3 (Cl, Br, I)

According to the Hume-Rothery rule, anions with a large difference in ionic radii, for example, chloride and iodide anions, cannot form solid solutions. In this study, we demonstrate that Ba2H3X (X = Cl, Br, or I) with the anti-Li3LaSb2-type structure, recently discovered to exhibit excellent hydride conductivity at low temperatures, can afford a trihalide solid solution containing a substantial amount of each halogen (e.g., Ba2H3Cl0.40Br0.35I0.25). The systematic study using Rietveld analysis and pair distribution function (PDF) analysis of synchrotron X-ray diffraction data revealed a significant deviation of the local structure from the average structural model for the solid solutions with fixed average radius but different size variances of X– anions. Analysis based on a triphasic model indicates that size-flexible H– anions play an important role in the formation of trihalide solid solutions, i.e., depending on the type of X, the HBa6 octahedron locally changes its size to accommodate the neighboring XBa6 octahedron. The chemical disorder in the hydride layer induced by the halide mixing reduces the activation barrier of the H– conduction, which is important for lowering operating temperatures. This study opens up the possibility to significantly expand the compositional space of anionic solid solutions by including hydride anion toward exploring novel functions.

Refined GFN1-xTB Parameters for Engineering Phase-Stable CsPbX3 Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications due to their excellent optoelectronic performance. However, they suffer from several dynamical degradation problems, the characterization of which is challenging. Atomic scale simulations can provide valuable insights; however, the high computational cost of first-principles methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present refined GFN1-xTB parameters for the accurate description of structural and dynamical properties of CsPbBr3, CsPbI3, and CsPb(I1–xBrx)3. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, directional movement of the Cs cations increases their distance to the surrounding halides, initiating a transition to the nonperovskite phase. However, once enough thermal energy is available, the increased Cs–halide distance can be compensated by increased halide fluctuations, resulting in a transition to the phase-stable tetragonal or cubic phases. Furthermore, we find the mixing of halides increases halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.

Halide sublattice dynamics drive Li-ion transport in antiperovskites

Proton incorporation and halide mixing allow for lower synthesis temperatures and more rapid processing of cubic lithium antiperovskites. Halide mixing softens halide-sublattice dynamics for improved Li+ transport.

Broadband-tunable spectral response of perovskite-on-paper photodetectors using halide mixing.

Paper offers a low-cost and widely available substrate for electronics. It possesses alternative characteristics to silicon, as it shows low density and high flexibility, together with biodegradability. Solution processable materials, such as hybrid perovskites, also present light and flexible features, together with a huge tunability of the material composition with varying optical properties. In this study, we combine paper substrates with halide-mixed perovskites for the creation of low-cost and easy-to-prepare perovskite-on-paper photodetectors with a broadband-tunable spectral response. From the bandgap tunability of halide-mixed perovskites we create photodetectors with a cut-off spectral onset that ranges from the NIR to the green region, by increasing the bromide content on MAPb(I1-xBrx)3 perovskite alloys. The devices show a fast and efficient response. The best performances are observed for pure I and Br perovskite compositions, with a maximum responsivity of ∼400 mA W-1 on the MAPbBr3 device. This study provides an example of the wide range of possibilities that the combination of solution processable materials with paper substrates offers for the development of low-cost, biodegradable and easy-to-prepare devices.

Halide Mixing Inhibits Exciton Transport in Two-dimensional Perovskites Despite Phase Purity.

Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA)2Pb(I1–xBrx)4. Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites.

Preparation and characterization of a Cu complex based on 2,2′-bipyrimidine as a recyclable metal-organic framework for Suzuki coupling

A Cu complex based on 2,2′-bipyrimidine ([CuBpm·2H2O]n) as a recyclable metal-organic framework (MOF) was used for the synthesis of biaryl derivatives by the Suzuki coupling reaction. The Suzuki reaction was performed with mixing of aryl halides with arylboronic acids in DMSO. The prepared catalyst was characterized by FT-IR, XRD, SEM, EDX, BET, BJH and ICP-AES analysis. [CuBpm·2H2O]n as catalyst demonstrated good to excellent yields for Suzuki reaction in comparison with other catalysts. The catalyst was recovered by a simple filtration and retained its activity even after several cycles.

Compositional Engineering of FAPbI3 Perovskite Added MACl with MAPbBr3 or FAPbBr3

Many attempts have been made to stabilize α-phase formamidinium lead iodide (α-FAPbI3) using mixed cations or anions with MA+, FA+, Br− and I−. A representative method is to stably produce α-FAPbI3 by adding methylammonium lead (MAPbBr3) to the light absorption layer of a perovskite solar cell and using methylammonium chloride (MACl) as an additive. However, in the perovskite containing MA+ and Br−, the current density is lowered due to an unwanted increase in the bandgap; phase separation occurs due to the mixing of halides, and thermal stability is lowered. Therefore, in this study, in order to minimize the decrease in the composition ratio of FAPbI3 and to reduce MA+, the addition amount of MACl was first optimized. Thereafter, a new attempt was made to fabricate FAPbI3 perovskite by using formamidinium lead bromide (FAPbBr3) and MACl together as phase stabilizers instead of MAPbBr3. As for the FAPbI3-MAPbBr3 solar cell, the (FAPbI3)0.93(MAPbBr3)0.07 device showed the highest efficiency. On the other hand, in the case of the FAPbI3-FAPbBr3 solar cell, the (FAPbI3)0.99(FAPbBr3)0.01 solar cell with a very small FAPbBr3 composition ratio showed the highest efficiency with fast photovoltaic performance improvement and high crystallinity. In addition, the FAPbI3-FAPbBr3 solar cell showed a higher performance than the FAPbI3-MAPbBr3 solar cell, suggesting that FAPbBr3 can sufficiently replace MAPbBr3.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites.

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of 1H, 13C, 15N, 14N, 133Cs, 87Rb, 39K, 207Pb, 119Sn, 113Cd, 209Bi, 115In, 19F and 2H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

Efficient Catalysts of Mixed Amino‐Functionalized Ionic Liquids and Zinc Halides for Cycloaddition of CO2to Propylene Oxide without Solvent

AbstractAmino‐functionalized halide ionic liquids, aminopropyl‐3‐butyl‐imidazolium ([Apbim]+) with different halogen anions, and their zinc halide mixing catalysts were prepared and used to catalyze the cycloaddition of CO2to propylene oxide (PO) without solvent. Under ideal conditions, the conversion of PO could reach 93.5 % with the catalysis of [Apbim]Cl. Under optimized conditions of catalyst dosage, reaction temperature, CO2pressure, and reaction time, with the catalysis of [Apbim]I, [Apbim]Cl/ZnBr2(3:1), and [Apbim]Br/ZnBr2(5:1), the conversion of PO and the selectivity of propylene carbonate (PC) both reach &gt; 99 %. From the point of view of preparation process and industrial operation, [Apbim]Cl/ZnBr2is the most suitable catalyst. This catalyst is characterized by good reusability and catalytic activity, and the yield of PC is about 99 % at least for ten times repeated use.

Improvement of the Resistive Switching Characteristics upon Halide Mixing in an All-Inorganic RbPbI3 Perovskite Polymer Composite Based Flexible Device

Improvement of the Resistive Switching Characteristics upon Halide Mixing in an All-Inorganic RbPbI₃ Perovskite Polymer Composite Based Flexible Device

Investigation of material properties of halide mixed lead - Free double perovskite for optoelectronic applications using first-principles study

All-inorganic halide double perovskite has recently emerged as a potential substitute for lead halide perovskite capable of solving the problem of toxicity associated with lead content and its inherent instability issue. However, most double perovskites possess wide bandgaps having drastic consequences on their efficiency, hindering their usage as solar absorbers in the photovoltaic industry. Halide mixing has been identified as a viable means of engineering bandgaps of all-inorganic halide double perovskite. This study investigates material properties of all-inorganic double perovskite Cs2NaM(IxBr1−x)6 [M=Bi,In] for optoelectronic applications. Structural, electronic, and optical properties of the materials are investigated using first-principles density functional theory with the help of virtual crystal approximation for modeling the virtual atoms IxBr1−x. The results show a linear increase in lattice parameters of the compounds as iodine content is increased while the bandgaps decrease following a quadratic function. For M=Bi, indirect band nature is observed while for M=In, direct band nature is observed all through the range. With increased iodine content, the optical absorption of the perovskites is observed to enhance weakly. The static dielectric constants increase quadratically with increased iodine content. Phase decomposition analysis of the materials shows that CNBX materials remain stable up to x=0.2 while CNIX remains stable up to x=0.6 against phase decomposition. Thus, halide mixing of iodine with bromine may be a viable means of bandgap reduction and enhancing optical properties of double perovskites for optoelectronic applications.

Study on A-Site Compositional Mixing for the Shear Coating Process of FA-Based Lead Halide Perovskites

Halide perovskite solar cells have been attracting tremendous attention as next-generation solar cell materials because of their excellent optical and electrical properties. Formamidinium lead tri-iodide (FAPbI3) exhibits the narrowest band gap among lead iodide perovskites and shows excellent thermal and chemical stability, also. However, the large-area coating of FAPbI3 needed for commercialization has not been successful because of the instability of the black phase of FAPbI3 at ambient temperature. This study presents a compositional engineering direction to control the polymorph of the FAPbI3 thin film for the shear coating processes, without halide mixing. By adopting a hot substrate above 100 oC, our shear coating process can produce the black phase FA-based halide perovskites without halide mixing. We carefully investigate the Cs-FA and MA-FA mixed lead iodide perovskites’ phase stability by combining the study with thin-film fabrication and ab initio calculations. Cs-FA mixing shows promising behaviors for stabilizing α-FAPbI3 (black phase) compared with MA-FA. Stable FA-rich perovskite films cannot be achieved via shear coating processes with MA-FA mixing. Ab initio calculations revealed that Cs-FA mixing is excellent for inhibiting phase decomposition and water incorporation. This study is the first report that FA-based halide perovskite thin films can be made with the shear coating process without MA-Br mixing. We reveal the origin of the stable film formation with Cs-FA mixing, and present future research directions for fabricating FA-based perovskite thin films using shear coating.

Stable cyan and white light-emitting diodes enabled by branched cations sterically stabilized 2D/3D perovskites

As an indispensable part of the visible light region, cyan emission (470 nm–500 nm) is of great importance in lighting, displays, and light communication. Metal halide perovskite materials have made significant progress in the field of luminescent materials and devices. However, the corresponding cyan emission through halide mixing is non-stable due to halogen migration and phase separation. Herein, we developed a steric stabilization approach to tune and stabilize the cyan emission of methylammonium (MA)-based perovskites. Dimethylammonium (DMA+) lead bromide, with the small DMA+ branched cation, was used as intermediate to react with MABr, forming blue-to-green emissive (MA)x(DMA)PbBr3+x compounds simular to the low-dimensional perovskites. Furthermore, protonated (trimethylsilyl)methylamine (TmMA), the larger branched cation, was adopted as an interlayer space to further reduce the perovskite dimension, tuning the emission to the cyan region. The combination of the two branched cations enabled us to achieve a champion photoluminescence quantum yield (PLQY) of ~87.8% (peak wavelength ~486 nm). Moreover, the as-synthesized double cations modified perovskites in combination with LED InGaN chip can be used to fabricate stable cyan emission LEDs with improved durability (over 1000 min) and white LEDs with increased color rendering index (from ~73.1 to ~81.7).

When iodide meets bromide: Halide mixing facilitates the light-induced decomposition of perovskite absorber films

APbX3 lead perovskites, where A is either an organic (methylammonium MA+ and formamidinium FA+) or an inorganic (Cs+) species, have recently emerged as highly promising photovoltaic materials. This interest is related to the exceptionally high power conversion efficiency (> 25%) demonstrated recently in the case of mixed-cation and mixed-halide perovskites. However, the poor intrinsic stability of complex lead halides remains a major hindrance in the commercialization of this emerging photovoltaic technology. An intense research effort is currently focused on revealing the origins and mechanistic aspects of the various pathways of degradation occurring in perovskite solar cells. In this paper, we present a systematic comparative study of a series of mixed-cation perovskite systems: MA0.15FA0.85PbI2.55Br0.45, Cs0.1MA0.15FA0.75PbI2.55Br0.45, Cs0.15FA0.85PbI2.55Br0.45, MA0.15FA0.85PbI3, Cs0.1MA0.15FA0.75PbI3, and Cs0.15FA0.85PbI3 all of which deliver high photovoltaic efficiencies in devices. Using a set of complementary analytical techniques, we demonstrate that bromide-containing mixed-halide perovskites have much lower photostability when compared to the equivalent iodide-based materials. The light-induced photochemical aging produced metallic lead as one of the final decomposition products in the case of all the studied complex lead halides except for Cs0.15FA0.85PbI3, which demonstrated outstanding stability under white light exposure. Theoretical calculations of the bromide-containing mixed-halide perovskites indicated that hole-coupling drives the formation of interstitial-vacancy halide pair defects to become more thermodynamically favorable, thus leading to the accelerated degradation of the halide-mixed perovskites. The obtained results demonstrate the usefulness of compositional engineering as a promising approach to boost the operational stability of perovskite solar cells and pave the way towards their successful commercialization.

Pb-free layered all-inorganic metal halides RbSn2Br5: Mechanochemical synthesis, band gap tuning, optical and dielectric properties

Recently, two-dimensional (2D) layered metal halide perovskites stimulate substantial attention in photovoltaics and optoelectronics. Nevertheless, one of the main hurdles to their commercialization is the Pb toxicity. Although challenging, the emergence of Sn(II)-based halides considered as a most favourable environmentally benign substitute to Pb based material. Herein, we investigate the optical and dielectric properties of 2D Pb-free all-inorganic metal halide, RbSn2Br5, which is synthesized by all-solid state mechanochemistry. The material exhibited a band gap of ∼3.20 eV with room-temperature photoluminescence (PL) centered at ∼2.51 eV. The Raman spectroscopic investigation demonstrated distinct vibrational modes related to the Sn-Br bond from 2D polymeric [Sn2Br5]nn− layer units. The thermal stability indicated that the material is stable up to ∼205 °C. An excitonic absorption with high exciton binding energy is perceived for ultrasonic treated toluene-dispersed solution of RbSn2Br5. Enhanced PL intensity with red-shifted and broad emission line were demonstrated at 77 K than the room-temperature emission. The charge-carrier recombination took place from the band-edge states and intrinsic defect sites (shallow states) at both temperatures. Further, the halide mixing strategy is applied to tune the band gap (2.68–3.36 eV) and PL emission (2.31–2.58 eV) by synthesizing RbSn2Br3Cl2 and RbSn2Br4I. Finally, the dielectric measurement of RbSn2Br5 was carried out as a function of temperature (4−300 K), which exhibited a strong frequency dependence.

Reversible Pb2+/Pb0 and I−/I3− Redox Chemistry Drives the Light‐Induced Phase Segregation in All‐Inorganic Mixed Halide Perovskites

AbstractTunability of optoelectronic properties of lead halide perovskites achieved through halide mixing can potentially enable their multiple applications, for example, in tandem solar cells and light‐emitting diodes. However, mixed halide perovskites are unstable under illumination due to their segregation to Br‐rich and I‐rich phases, which negatively affects the performance and the operational stability of devices. Research efforts over the past years provided a substantial understanding of the factors influencing light‐induced halide phase segregation. While several mechanisms have been proposed, none of them could account for all available experimental data; and hence the origin of the effect is still under active debate. Herein, the photodegradation of CsPbI2Br and Cs1.2PbI2Br1.2 is thoroughly investigated using a set of complementary techniques. In situ atomic force microscopy provides a visualization of the real‐time halide phase segregation dynamics demonstrating that iodoplumbate is selectively expelled from the mixed halide perovskite grains and nucleates as a separate I‐rich phase at the grain boundaries. A mechanism based on the reversible Pb2+/Pb0 and I−/I3− redox (photo)chemistry is proposed, which explains the experimental findings and other previously reported results. Furthermore, it sheds new insights on the underlying mechanisms of multiple phenomena related to light‐ or electric field‐induced degradation of various lead halide perovskites.

Defect suppression and photoresponsivity enhancement in methylammonium lead halide perovskites by CdSe/ZnS quantum dots

Potential strategies such as surface passivation and perovskite material halide mixing may protect material surfaces, improve luminescence, and reduce charge traps for device stability. In this study, we used deep level transient spectroscopy to investigate the effect of CdSe/ZnS core-shell quantum dots (QDs) on defect states and carrier transport in methylammonium (MA) lead halide perovskites (CH3NH3PbX3 where X = I, Br). In MAPbI3 and MAPbI2Br films with CdSe/ZnS QDs, the density of hole traps located at Ev + 0.37 eV and Ev + 0.56 eV was reduced dramatically. Deep traps at Ev + 0.78 eV and Ev + 1.08 eV were removed, and one broad electron trap signal dominated. Film photoresponsivity under 600-nm wavelength light and a bias voltage of −0.7 V was 10 and 18 mA/W, which is 100 and 27 times larger than the 0.1 and 0.67 mA/W of bare perovskites (PS), respectively. This demonstrates that carrier transport was enhanced due to defect suppression. Our findings on defect suppression and photoresponsivity enhancement provide an important direction for optimizing high-performance PS device fabrication.

Synthesis of brightly luminescent colloidal formamidinium lead bromide perovskite FAPbBr3 nanoplatelets with tunable emission

Hybrid halide perovskites are semicondoctor materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology. They have size-tunable emissions due to quantum size effects. In this work, the Formamidinium Lead Bromide perovskite CH(NH2)2PbBr3 nanoplatelets (NPLs) were successfully synthesized by ligand-assisted reprecipitation method under room condition, in which the emission color-tunability was realized via quantum size effect without anion–halide mixing, by varying the oleylamine to oleic acid volume ratio as surfactants, while the total amount of oleic acid remained unchanged. We are able to adjust the optical proprieties of FAPbBr3 NPLs and, consequently, their structural properties. The obtained colloidal solutions of FAPbBr3 nanoplatelets with uniform size exhibited different photoluminescence wavelengths covering the spectral region from 440 to 525 nm. The maximum absolute PL quantum yield (PLQY) of the green emission was measured to be as high as 80% at room temperature. The size of FAPbBr3 NPLs could be effectively tuned from 15.5 to 38.1 nm with an increase in the oleylamine and oleic acid ligands ratio.